Traveling-wave tube having loss-filled, capacitively-coupled cavities coupled to the interaction cells of the slowwave structure



Nov. 21, 1967 A. LAVlK 3,354,346

TRAVELING-WAVE TUBE HAVING LOSS-FILLED, CAPACITIVELY-COUPLED CAVITIESCOUPLED TO THE INTERACTION CELLS OF Filed Oct. 23, 1964 THE SLOW-WAVESTRUCTURE 3 Sheets-Sheet 1 ZZZ-61.2.

WVlM/UZ. A a 44144 5 4 Nov. 21. 1967 A. LAVIK v3,354,346

TRAVELING-WAVE TUBE HAVING LOSS-FILLED. CAPACITIVELY-COUPLED CAVITIESCOUPLED TO THE INTERACTION CELLS OF THE SLOW-WAVE STRUCTURE Filed Oct.23, 1964 5 Sheets-Sheet 5 United States Patent TRAVELING-WAVE TUBEHAVING LOSS-FILLED, CAPACITIVELY-COUPLED CAVITIES COUPLED TO THEINTERACTION CELLS OF THE SLOW- WAVE STRUCTURE Arne Lavik, Hawthorne,Califi, assignor to Hughes Aircraft Company, Culver City, Calif acorporation of Delaware Filed ()ct. 23, 1964, Ser. No. 406,120 6 laims.(Cl. 3153.5)

This invention relates generally to microwave devices, and moreparticularly relates to traveling-wave tubes having novel and improvedmeans for substantially eliminating oscillations at desired frequencies,such as those at the edges of the frequency passband of the tube.

In traveling-wave tubes a stream of electrons is caused to interact witha propagating electromagnetic wave in a manner which amplifies theelectromagnetic energy. In order to achieve such interaction, theelectromagnetic Wave is propagated along a slow-wave structure, such asa conductive helix wound about the path of the electron stream or afolded waveguide type of structure in which a waveguide is effectivelywound back and forth across the path of the electrons. The slow-wavestructure provides a path of propagation for the electromagnetic wavewhich is considerably longer than the axial length of the structure, andhence, the traveling-wave may be made to effectively propagate at nearlythe velocity of the electron stream. The interactions between theelectrons in the stream and the traveling-wave cause velocitymodulations and bunching of the electrons in the stream. The net resultmay then be a transfer of energy from the electron beam to the wavetraveling along the slow-wave structure.

The present invention is primarily, although not necessarily, concernedwith traveling-wave tubes utilizing slowwave structures of the coupledcavity, or interconnected cell, type. In this type of slow-wavestructure a series of interaction cells, or cavities, are disposedadjacent to each other sequentially along the axis of the tube. Theelectron stream passes through each interaction cell, andelectromagnetic coupling is provided between each cell and the electronstream. Each interaction cell is also coupled to an adjacent cell bymeans of a coupling hole at the end wall defining the cell. Generally,the coupling holes between adjacent cells are alternately disposed onopposite sides of the axis of the tube, although various otherarrangements for staggering the coupling holes are possible and havebeen employed. When the coupling holes are so arranged, a foldedwaveguide type of energy propagation results, with the traveling-waveenergy traversing the length of the tube by entering each interactioncell from one side, crossing the electron stream and then leaving thecell from the other side, thus traveling a sinuous, or serpentine,extended path.

One of the problems encountered in traveling-wave tubes of the coupledcavity variety, and especially high power tubes of this type, is atendency for the tube to oscillate at frequencies near the edges of thetube passband. This problem arises from the fact that for wide bandoperation the phase velocity of the slow-wave circuit wave and thevelocity of the electron beam should be essentially synchronized over aslarge a range of frequencies as possible; hence, these velocities arealso close to synchronism near the upper and lower cutoff frequencies ofthe tube. Since the interaction impedance is high and thecircuit-to-transmission line match is poor at and in the vicinity of thecutoff frequencies, the loop gain for the tube, or even for a section ofthe tube, may be sufficiently large for oscillations to start.

One technique which has been used to solve this oscillation probleminvolves coupling to the slow-wave structure interaction cells speciallydesigned cavities which are sharply resonant at a frequency in thevicinity of a cutoff frequency of the slow-wave structure and providinglossy ceramic buttons in these special cavities in order to attenuateenergy at the resonant frequency of the cavity. Each lossy resonantcavity has a relatively high Q and thus provides a large amount of lossover a narrow frequency band. In order to obtain a sufficiently widebandwidth for the attenuation it has been the practice to tunerespective loss cavities in a portion of the tube to slightly differentfrequencies throughout the desired attenuation frequency band. Thistuning may be accomplished by changing the dimensions of either thelossy resonant cavities or the irises which coupled the loss cavities tothe slow-wave circuit interaction cells, or both, by altering the lossbutton material, or by loading the loss cavities with conductive stubsof varying dimensions. It will be apparent that such a cavity tuningprocedure requires considerable time and effort, thereby increasing themanufacturing time and complexity, and hence, the cost of thetraveling-wave tube.

Accordingly, it is an object of the present invention to provide ahigh-power wide-bandwidth traveling-wave tube having means forpreventing oscillations at the edges of the frequency passband of thetube, and which oscillation preventing means lends itself to the ready,rapid, efficient, and inexpensive manufacture of the traveling-wavetube.

It is a further object of the present invention to provide atraveling-wave tube of the type employing lossy resonant cavities foroscillation suppression and in which the amount of loss and the Q of thelossy cavities are more readily controllable than in the prior art.

It is a still further object of the present invention to provide atraveling-wave tube which employs lossy resonant cavities to attenuateenergy at frequencies at the edges of the slow-wave circuit frequencypassband, and which lossy resonant cavities provide more stableattenuation than in the prior art.

It is still another object of the present invention to provide a lossyresonant cavity oscillation suppression arrangement for a traveling-wavetube which affords a greater loss-bandwidth product than comparableprior art arrangements.

In accordance with the foregoing objects, the travelingwave tube of thepresent invention includes means for providing a stream of electronsalong a predetermined path and a slow-wave structure having a pluralityof intercoupled interaction cells disposed sequentially along and aboutthe electron stream path for propagating electromagnetic wave energy insuch manner that it interacts with the stream of electrons. A pluralityof cavities are sequentially disposed along a direction parallel to theelectron stream path, with each cavity being electromagnetically coupledto one of the interaction cells and being resonant at a preselectedfrequency. Loss means is disposed in each of the cavities forattenuating electromagnetic wave energy at the resonant frequency of thecavity. Wall means separating each pair of adjacent resonant cavitiesdefine an aperture for providing capacitive coupling directly betweenthe pair of resonant cavities in order to afford the improved loss vs.frequency characteristics of the present invention.

The foregoing, as well as other objects, advantages, and characteristicfeatures of the present invention will become more readily apparent fromthe following detailed description of a preferred embodiment of theinvention when considered in conjunction with the accompanying drawingsin which:

FIGJl is an overall view, partly in longitudinal section and partlybroken away, of a travelingwave tube constructed in accordance with thepresent invention;

FIG. 2 is a cross-sectional view taken along line 22 of FIG. l;

FIG. 3 is a longitudinal sectional view taken along line 33 of FIG. 2;

FIG. 4 is a longitudinal sectional view taken along line 4--4 of FIG. 2;and

FIG. 5 is a series of graphs illustrating the attenuation as a functionof frequency for traveling-wave tube lossy resonant attenuating devicesaccording to both the prior art and the present invention.

Referring to the drawings with more particularity, in FIG. 1 thereference numeral designates generally a traveling-wave tube whichincludes an arrangement 12 of magnets, pole pieces and spacer elementswhich will be described in detail later. At this point it should sufficeto state that the spacer elements and interior portions of the polepieces function as a slow-wave structure, while the magnets and polepieces constitute a periodic focusing device for the electron beamtraversing the length of the slow-wave structure.

Coupled to the input end of the arrangement 12 is an input waveguidetransducer 14 which includes an impedance step transformer 16. A flange18 is provided for coupling the assembled traveling-wave tube '10 to anexternal waveguide or other microwave transmission line (not shown). Theconstruction of the flange 18 may include a microwave window (not shown)transparent to microwave energy but capable of maintaining a vacuumwithin the traveling-wave tube 10. At the output end of the arrangement12 an output transducer 20 is provided which is substantially similar tothe input transducer 14 and which includes an impedance step transformer22 and a coupling flange 24, which elements are similar to the elements16 and 18, respectively, of the input transducer 14. For vacuum pumpingor out-gassing the travelingwave tube 10 during manufacture, adouble-ended pumping tube 26 is connected to both of the input andoutput waveguide transducers 14 and 28.

An electron gun 28 is disposed at one end of the traveling-wave tube 10which, although illustrated as the input end in FIG. 1, mayalternatively be the output end if a backward wave device is desired.The electron gun 28 functions to project a stream of electrons along theaxis of the tube 10 and may be of any conventional construction wellknown in the art. For details as to the construction of the gun 28reference is made to Patent No. 2,985,791, entitled, PeriodicallyFocused Severed Traveling-Wave Tube, issued May 23,1961, to D. J. Bateset al. and assigned to the assignee of the present invention and toPatent No. 2,936,393, entitled, Low Noise Traveling- Wave Tube, issuedMay 10, 1960, to M. R. Currie et al. and assigned to the assignee of thepresent invention.

At the output end of the traveling-wave tube 10 there is provided acooled collector structure 30 for collecting the electrons in thestream. The collector is conventional and may be of any form well knownin the art. For details as to the construction of the collector,reference is made to the aforesaid Patent No. 2,985,791 and to PatentNo. 2,860,277, entitled, TravelingWave Tube Collector Electrode, issuedNov. 11, 1958, to A. H. Iversen and assigned to the assignee of thepresent invention.

The construction of the slow-wave structure and magnetic focusing systemfor the traveling-wave tube 10 are illustrated in more detail in FIGS.2-4. A plurality of essentially annular disk-shaped focusing magnets 32are interposed between a plurality of ferromagnetic pole pieces 34. Asis illustrated in FIG. 2, the magnets 32 may be diametrically split intotwo sections 3211 and 32b for convenience during assembly of the tube.The ferromagnetic pole pieces 34 extend radially inwardly of the magnets32 to approximately the perimeter of the region adapted to contain theaxial electron stream. The individual pole pieces are constructed insuch a manner that a short drift tube, or ferrule, 36 is provided at theinner extremity of each pole piece. The drift tube 36 is in the form ofa cylindrical extension, or lip, protruding axially along the path ofthe electron stream from both surfaces of pole piece 34, i.e., in bothdirections normal to the plane of the pole piece 34. The drift tubes 36are provided with central and axially aligned apertures 38 to provide apassage for the flow of the electron beam. Adjacent ones of the drifttubes 36 are separated by a gap 46 which functions as a magnetic gap toprovide a focusing lens for the electron beam and also as an interactiongap in which energy exchange between the electron beam andtraveling-wave energy traversing the slow-wave structure occurs.

Disposed radially within each of the magnets 32 is a slow-wave circuitspacer. element 42 of a conductive nonmagnetic material such as copper.Each spacer element 42 has an annular portion of an outer diameteressentially equal to the inner diameter of the magnets 32 and a pair ofoppositely disposed ear portions 43 and 44 projecting outwardly from theannular portion. Each spacer element also defines a central cylindricalaperture 45 to provide space for a microwave interaction cell, orcavity, 46 which is defined by the inner lateral surface of the spacer42 and the walls of the two adjacent pole pieces 34 projecting inwardlyof the spacer element 42. The inner diameter of the spacer 42 determinesthe radial extent of the interaction cell 46, while the axial length ofthe spacer 42 determines the axial length of the cell 46.

For interconnecting adjacent interaction cavities 46 an off-centercoupling hole 48 is provided through each of the pole pieces 34 topermit the transfer of electromagnetic wave energy from cell to cell. Asis illustrated, the coupling holes 48 may be substantially kidney'shapedand may be alternately disposed apart with respect to the drift tubes36. It should be pointed out, however, that the coupling holes 48 may beof other shapes and may be staggered in various other arrangements, suchas those disclosed in Patent No. 3,010,047, entitled Traveling-WaveTube, issued Nov. 21, 1961 to D. J. Bates and assigned to the assigneeof the present invention. In any event, it will be apparent that thespacer elements 42 and the portions of the pole pieces 34 projectinginwardly of the spacers 42 not only form an envelope for the tube, butalso constitute a slow-wave structure for propagating traveling-waveenergy in a serpentine path along the axially traveling electron streamso as to support energy exchange between the electrons of the stream andthe traveling-wave.

The axial length of the magnets 32, hence that of the spacers 42, isequal to the spacing between adjacent pole pieces 34, and the radialextent of the magnets 32 is approximately equal to or, as shown,slightly greater than that of the pole pieces 34-. To provide focusinglenses in the gaps 40, the magnets 32 are stacked with alternatingpolarity along the axis of the tube, thus causing a reversal of themagnetic field at each magnetic lens and thereby providing a periodicfocusing device. It should be pointed out, however, that although thelengths of the spacers 42 may be substantially constant, they may alsobe varied slightly with respect to each other so that the effectiveaxial length of the cavities 46 is varied as a function of distancealong the tube to ensure that the desired interaction between theelectron stream and the traveling waves will continue to a maximumdegree even though the electrons are decelerated toward the collectorend of tube.

In order to minimize any tendency for the travelingwave tube tooscillate at frequencies near the edges of the slow-wave circuitpassband, frequency selective attenuation is provided to substantiallydecrease the gain at these frequencies and, thereby, suppress theoscillations. This attenuation takes the form of lossy ceramic elementsdisposed in cavities which are coupled to the slow-wave circuitinteraction cells and which cavities are made resonant at thefrequencies to be attenuated. Thus, as is shown in FIGS. 2 and 4, aslow-wave circuit spacer element 42 may define a pair of cylindricalcavities 50 and 52 which are respectively disposed in the projecting earportions 43 and 44 of the spacer element 42. The cavity 50 has adiameter d and is coupled to the central aperture 45 in the spacer 42 bymeans of a coupling hole, or iris, 54 of width i Similarly, the cavity52 has a diameter d and is coupled to the spacer aperture 45 via acoupling iris 56 of width i The diameters d and d for the respectivecavities 50 and 52 may either have the same or a different value, andsimilarly, the iris widths i and i may or may not be equal. As may beseen from FIG. 4, the cavities 50 and 52 have a length 1 equal to thethickness of the slow-wave circuit spacer element 42. The cavities 50and 52 are designed to resonate in the TM mode at a frequency at whichloss is to be introduced into the circuit. Although the cavity resonantfrequency is preferably at or near either the upper or lower cut-offfrequency of the slow-wave circuit, it is understood that the resonantloss frequency may be any preselected frequency.

Cylindrical button-like elements 57 and 58 of a mixture of ceramic andlossy materials are disposed in the respective cavities 50 and 52 inorder to provide the desired loss. A composition which may be used forthe buttons 57 and 58 is a mixture of forsterite and silicon carbide,with the percentage of silicon carbide being esentially between 3% andExamples of other materials which could be used are silicon carbide andalumina, silicon carbide and talc, or other ceramic and lossy materialcombinations.

For the TM mode, the cavity resonant frequency is determined by thediameter of the cavity and the dielectric constant of the lossymaterial. However, since the normal TM cylindrical cavity mode isperturbed by the relatively large irises 54 and 56 which are designed toprovide critical coupling into the respective cavities 50 and 52, thecavity resonant frequency also becomes a function of the irisdimensions. Thus, the cavity diameter d, the iris width i, and thedielectric constant of the lossy material in the buttons 57 and 58 mustbe varied dependently to achieve the desired attenuation at the desiredfrequency. In addition, it should be pointed out that as the compositionof the lossy ceramic mixture is changed the Q of the resonance will beaffected, i.e., as the percentage of silicon carbide is increased the Qwill decrease.

In prior art resonant loss arrangements the pole pieces 34 provide solidend walls for the resonant cavities 50 and 52, and thus the onlycoupling with the cavities 50 and 52 is through the respective irises54-and 56 to the slow-wave circuit interaction cells 46. In sucharrangements each lossy resonant cavity has a relatively high Q andhence provides a large amount of attenuation over a relatively narrowband of frequencies. Since this frequency band may be of insufficientwidth to afford the desired oscillation suppression, a stagger tuningprinciple has been employed. For example, different resonant losscavities are tuned to slightly different frequencies near the slow-wavestructure upper cutoff frequency so as to introduce loss in contiguousor even overlapping narrow frequency bands in the vicinity of the uppercutoff frequency, thereby forming a composite loss band of greater widththan the individual loss bands provided by the respective resonantcavities.

In accordance with the present invention a widening of the resonant lossband is afforded without staggertuning the individual cavities and in amanner which insures a stable attenuation vs. frequency characteristicregardless of changes in environmental conditions such as temperature.These advantageous results are achieved by providing capacitive couplingdirectly between axially adjacent ones of the resonant loss cavities 50or 52. For

this purpose cylindrical coupling apertures 60 of a diameter 0 areprovided in the pole pieces 34 between adjacent cavities 50 on one sideof the slow-wave structure, and similar coupling apertures 62 having adiameter 0 are provided in the pole pieces between adjacent cavities 52on the other side of the slow-wave structure. The coupling apertures 60are coaxially aligned with the cylindrical cavities 50, while theapertures 62 are coaxially aligned with the cavities 52. Preferably, thediameters c and c of the coupling holes 60 and 62, respectively, varyfrom essentially between 0.2 and 0.7 of the diameters (I and d of theresonant cavities 50 and 52. The length of the coupling apertures 60 and62, which is equal to the thickness of the pole piece 34 defining theapertures, is denoted by s in FIG. 4. The diameters c and 0 for therespective coupling apertures 60 and 62 may or may not be equal.

The manner in which the resonant loss cavity capacitive couplingarrangement of the present invention affects the attenuation vs.frequency characteristics of the slowwave structure is illustrated inFIG. 5. In this figure the dashed curve 70 depicts the attenuation as afunction of frequency for a prior art scheme in which no capacitivecoupling is provided between axially adjacent resonant loss cavities. Inthe exemplary arrangement from which data for the curve 70 was taken thediameters d and d of the resonant cavities 50 and 52 were both .150inch, the coupling iris widths i and i were both .130 inch, the cavitylength l was .096 inch, the pole piece thickness s was .040 inch, thecoupling aperture diameters c and c were both zero, and the loss buttons57 and 58 each contained 3 /2% silicon carbide and 96 /z% forsterite.

Attenuation vs. frequency characteristics for resonant loss arrangementsin accordance with the present invention are illustrated by the solidcurves 72 and 74 of FIG. 5. The curve 72 was made from a slow-wavecircuit-resonant loss arrangement having parameters identical to thoseset forth above with respect to the curve 70, except that cylindricalcoupling apertures 60 and 62 were provided in the pole pieces 34 betweenadjacent resonant loss cavities 50 and 52, respectively, the diameters cand 0 for the coupling apertures 60 and 62 each being .075 inch. Thecurve 74 was made from an arrangement identical to that from which thecurve 72 was plotted, except with a coupling aperture diameter c =c=.O90 inch.

From inspection of FIG. 5, it will be apparent that as the couplingaperture diameter increases the bandwidth of the attenuation bandincreases substantially, the center frequency of the attenuation bandincreases somewhat, and the amplitude of the maximum attenuationdecreases very slightly. Thus, the loss-bandwidth product of theresonant loss arrangement of the present invention may be seen to besubstantially greater than an arrangement of the prior art otherwisehaving the same parameters. It will also be apparent from FIG. 5 thatboth the amount of loss and the Q of the lossy resonant cavities may becontrolled by varying the diameter of the capacitive coupling holes 60and 62, thereby affording a further and more workable way to controlthese parameters than in the prior art. Moreover, a sufficiently wideloss band may be introduced without the necessity for stagger-tuning theindividual resonant loss cavities, thereby decreasing manufacturing timeand complexity, and hence, reducing the cost of the traveling-wave tube.In addition, since the coupled lossy resonant cavity arrangement of thepresent invention eliminates the necessity for forming the desired wideloss band with a plurality of individual narrower loss bands, anypossibility of the formation of gaps in the attenuation vs. frequencycharacteristic due to resonance shifting of an individual loss button asa function of temperature or other environmental changes is precluded,thereby affording more stable attenuation than in the prior art.

Although the present invention has been shown and described withreference to a particular enbodiment, nevertheless, various changes andmodifications obvious to a person skilled in the art to which theinvention pertains are deemed to be within the spirit, scope andcontemplation of the invention as set forth in the appended claims.

What is claimed is:

1. A traveling-wave tube comprising: means for providing a stream ofelectrons along a predetermined path, slow-wave structure means defininga plurality of intercoupled interaction cells disposed sequentiallyalong and about said predetermined path for propagating electromagneticwave energy in such manner that it interacts with said stream ofelectrons, means defining a plurality of cavities sequentially disposedalong a direction parallel to said predetermined path, each of saidcavities being electromagnetically coupled to one of said interactioncells and being resonant at a preselected frequency, means for providingcapacitive coupling directly between adjacent ones of said cavities, andloss means disposed in each of said cavities for attenuatingelectromagnetic wave energy at the resonant frequency of the cavity.

2. A traveling-wave tube comprising: means for providing a stream ofelectrons along a predetermined path, slow-wave structure means defininga plurality of intercoupled interaction cells disposed sequentiallyalong and about said predetermined path for propagating electromagneticwave energy in such manner that it interacts with said stream ofelectrons, means defining a plurality of sequentially disposedcylindrical cavities aligned with one another along a direction parallelto said predetermined path, each of said cavities beingelectromagnetically coupled to one of said interaction cells and beingresonant at a preselected frequency, said cavity defining meansincluding electrically conductive wall means separating adjacent ones ofsaid cavities, said wall means defining a cylindrical aperture coaxiallyaligned with said cylindrical cavities for providing electromagneticcoupling between said adjacent ones of said cavities, and loss meansdisposed in each of said cavities for attenuating electromegnatic waveenergy at the resonant frequency of the cavity.

3. A traveling-wave tube according to claim 2 wherein said loss meanscomprises an element of a mixture of silicon carbide and a materialselected from the group consisting of forsterite, alumina and talc, withthe percentage of silicon carbide being essentially between 3% and 10%.

4. A traveling-wave tube according to claim 2 wherein each of saidcavities is resonant in the TM mode.

5. A traveling-wave tube comprising: means for providing a stream ofelectrons along a predetermined path, a plurality of axially alignedessentially annular electrically conductive spacer elements sequentiallydisposed along and encompassing said predetermined path, a plurality ofelectrically conductive plates each mounted between a pair of adjacentspacer elements to define in conjunction with said spacer elements aplurality of interaction cells, said plates defining aligned aperturesin their central regions to provide a passage for said electron streamand further defining coupling holes in regions readily outwardly of saidcentral regions for interconnecting adjacent interaction cells whereby apropagation path is provided for an electromagnetic wave in a manner toprovide interaction between said electron stream and saidelectromagnetic wave, at least certain successive ones of said spacerelements defining aligned cylindrical cavities coupled to the respectiveinteraction cells defined by the spacer elements, each of said cavitiesbeing resonant at a preselected frequency, each of said plates which isinterposed between a pair of successive cylindrical cavities defining acylindrical aperture intercoupling said pair of cylindrical cavities,and loss means disposed in each of said cylindrical cavities forattenuating electromagnetic energy at the resonant frequency of thecavity.

6. A traveling-wave tube comprising: means for launching a stream ofelectrons along a predetermined path, a plurality of axially alignedessentially annular magnets, a plurality of ferromagnetic pole piecesinterposed between and abutting adjacent magnets, a hollow essentiallycylindrical nonmagnetic spacer element having an'outer diameteressentially equal to the inner diameter of said essentially annularmagnets disposed within each of said magnets, said pole piecesprojecting internally of said spacer elements to define therewith aplurality of interaction cells, said pole pieces defining alignedapertures in their central regions to provide a passage for saidelectron stream and further defining coupling holes in regions readilyoutwardly of said central regions for interconnecting adjacent cellswhereby a propagation path is provided for an electromagnetic wave in amanner to provide interaction between said electron stream and saidelectromagnetic wave, at least certain successive ones of said spacerelements each defining at least one outwardly extending ear portion, atleast certain successive ones of said ear portions defining alignedcylindrical cavities coupled tothe respective interaction cells definedby the spacer elements, each of said cavities being resonant at apreselected frequency, each of said plates which is interposed between apair of successive cylindrical cavities defining a cylindrical aperturecoaxially aligned with and intercouplingsaid pair of cylindricalcavities, and loss means disposed in each of said cylindrical cavitiesfor attenuating elecromagnetic energy at the resonant frequency of thecavity.

References Cited UNITED STATES PATENTS,

3,221,204 11/1965 Hant et al. 3153.5

HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner,

1. A TRAVELING-WAVE TUBE COMPRISING: MEANS FOR PROVIDING A STREAM OFELECTRONS ALONG A PREDETERMINED PATH SLOW-WAVE STRUCTURE MEANS DEFININGA PLURALITY OF INTERCOUPLED INTERACTION CELLS DISPOSED SEQUENTIALLYALONG AND ABOUT SAID PREDETERMINED PATH FOR PROPAGATING ELECTROMAGNETICWAVE ENERGY IN SUCH MANNER THAT IT INTERACTS WITH SAID STREAM OFELECTRONS, MEANS DEFINING A PLURALITY OF CAVITIES SEQUENTIALLY DISPOSEDALONG A DIRECTION PARALLEL TO SAID PREDETERMINED PATH, EACH OF SAIDCAVITIES BEING ELECTROMAGNETICALLY COUPLED TO ONE OF SAID INTERACTIONCELLS AND BEING RESONANT AT A PRESELECTED FREQUENCY, MEANS FOR PROVIDINGCAPACITIVE COUPLING DIRECTLY BETWEEN ADJACENT ONES OF SAID CAVITIES, ANDLOSS MEANS DISPOSED IN EACH OF SAID CAVITIES FOR ATTENUATINGELECTROMAGNETIC WAVE ENERGY AT THE RESONANT FREQUENCY OF THE CAVITY.